Isolation is required between a source of alternating current power and sensitive electronic components, such as logic circuitry, to prevent electronic system malfunctions from EMP induced upset and/or burnout of the electronic components arising from EMP sources such as a nuclear detonation. It is known to use filters and surge arrestor devices, such as spark gaps and zener diodes, with transformers for isolation on power lines and on low frequency lines. With capacitive and/or inductive filters, the isolation afforded is dependent on source or load impedance, and saturation of the transformer core during a high current pulse degrades isolation. Furthermore, with such filters there is a possibility of high voltage arcing from the transformer primary to the secondary, high power factors, and increased weight and decreased reliability due to the presence of additional electrical components. The use of spark gaps also imposes additional space and weight penalties, and zener diodes can be burned out by a high current pulse.
A previously patented isolation transformer for protecting sensitive electronic equipment from voltage excursions caused by electromagnetic and electrostatic interference (which is typically below 1 MHz.) includes a metallic shield disposed between the primary winding and the secondary winding. This shield, formed by overlapping plates, fits within the transformer core and has extending ends which are enclosed by the end bells of the transformer. The end bells are provided with mounting legs for use in attachment of the transformer to supporting structure. For further information concerning the structure and operation of this transformer, reference may be made to U.S. Pat. No. 4,484,171.